Weekly Emacs tip #20: Emacs window management — quickly moving to the right window with ace-window

In the previous three Emacs tips, I discussed various ways to select a window:

• other-window bound to the painful C-x o, which cycles from one window to the next;
• setting the mouse-autoselect-window variable to t for “focus follows mouse”;
• the windmove package that allows you to use a key prefix (super ctrl in my config) with the arrow keys to move the focus from one window to another adjacent one; and another prefix (super ctrl meta) + arrow keys to move a buffer (the contents of a window) from one window to an adjacent one.

In this tip, I’ll cover the ace-window package, which ended up being the way I most often switch between windows, or kill them.

The way ace-window works is rather simple: load the package and bind a key to the ace-window command (I use M-o as suggested in the package’s README). Then, when you press that shortcut, a “special” character appears in each window’s top left corner. Next, press the character’s key and there you are: that window is now active.

In the screenshot below, you see the three big blue letters a, s, d in the three buffers. When I pressed M-o, my cursor was in the top right window marked by s, in fact, you can still see the white rectangle next to that letter. Now, if I would press a, I would switch to the Org mode buffer with the text of this post.

Figure 1: Three Emacs windows before using ace-window.

The two main things I like about ace-window are its simple shortcuts —which is much easier to press than either the one for other-window or any of the windmove key combos— and the fact that I don’t need to think about “direction”. I just look at the window I intend to switch to, press M-o, see the blue letter and press it. That’s all there is to it.

This is how I load and configure the package in my ~/.emacs file:

;;; Window switching using ace-window
;;; https://github.com/abo-abo/ace-window
(use-package ace-window
  :bind
  ("M-o" . ace-window)
  ("M-O" . ace-delete-window)
  :custom
  ;; Don't cycle through other frames, stick to the current one.
  (aw-scope 'frame)
  ;; Don't use numbers to label the windows, but the keys on home row.
  (aw-keys '(?a ?s ?d ?f ?g ?h ?j ?k ?l))
  :config
  ;; Set the colour and size of the font used to display the selection
  ;; characters
  (set-face-attribute 'aw-leading-char-face nil
                      :foreground "deep sky blue"
                      :weight 'bold
                      :height 3.0)
  )

In the :bind section, you bind Alt with the capital O to ace-delete-window, which means I can easily delete a window that I’m not using, without the need to first switch to it. Very powerful! The aw-keys line sets the letters ace-window should show. Here, I set them to the keys on home row so I can quickly press them, without the need to move my fingers or hands too much. I haven’t run into a case where I had more windows than these letters could cover.

The set-face-attribute line sets font attributes (a “face” is Emacs speak for a font) for the selection letters: the colour and weight and size. You can, of course adjust these to your taste and theme.

There’s one more thing to note about ace-window, and that is that it allows you to do more than just switching windows. If you press ? after pressing M-o, you’ll see a list of one-key actions. For example, x will kill the window whose letter you select, m will swap the contents of the current window with the one corresponding to the key you press, and F will split the window fairly into two windows. I don’t use these very often as they aren’t (yet?) part of my muscle memory, but it is good to know they exist. Maybe they’ll replace the C-x 0, 1, 2, 3, keybindings (and more) one day.

And, as a last point: ⁠Karthink’s blog post on Emacs window management has a section on ace-window too (and ⁠more!).